1. Field of the Invention:
The present invention relates to processes and catalysts for transforming methane to higher hydrocarbons.
2. Discussion of the Background:
It is the business of many refineries and chemical plants to obtain, process and upgrade relatively low value hydrocarbons to more valuable feeds, or chemical raw materials. Methane, the simplest of the saturated hydrocarbons, is often available in large quantities either as an undesirable by-product in admixture with other more valuable higher molecular weight hydrocarbons, or as a component of an off-gas from a process unit, or units. This methane however, is not fully utilized by the chemical industry because it is expensive to transport and is not readily transformed into a derivative which could be easily transported. Enormous amounts of this natural resource are wasted or left lying unused in remote storage sites due to prohibitive transportation costs.
Natural gas which contains high concentrations of methane is produced in considerable quantities in oil and gas fields, often at remote locations and in difficult terrains, e.g., off-shore sites, artic sites, swamps, deserts and the like. Under such circumstances the natural gas is often flared while the oil is recovered, or the gas is shut in, if the field is too remote for the gas to be recovered on a commercial basis. The construction of pipelines to carry the gas is often not economical, due particularly to the costs of connnecting numerous well sites with a main line.
This problem has been addressed in several ways. One approach has been to liquify the natural gas and transport the liquid natural gas in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume it occupies in the gaseous state by cryogenic processing, and with proper procedures, safely stored or transported. Transport of natural gas under such circumstances is uneconomical however because methane at atmospheric pressure boils at -258.degree. F. and transportation economics dictate that the gas be liquifiable at substantially atmospheric pressures to reduce its volume.
Another approach to this problem has been the conversion of natural gas to other higher hydrocarbons that can be easily handled and transported, preferably substantially liquid hydrocarbons. The conversion of natural gas to higher order hydrocarbons, especially ethane and ethylene, would retain the material's versatility for use as precursor materials in chemical processing.
Known dehydrogenation and polymerization processes are available for the further conversion of ethane and ethylene to liquid hydrocarbons. In this way, easily transportable commodities may be derived directly from natural gas at the wellhead. A drawback in implementing such processes however has been in obtaining an efficient process for converting natural gas to higher order hydrocarbons.
The utilization of methane has been identified and targeted as one of the most important challenges facing the catalysis community today. This emphasis is not mislaid since methane represents a considerable source of energy and carbon atoms which could be used to assemble larger hydrocarbons or their derivatives.
Methane is the principal component of natural gas, which is composed of an admixture of normally gaseous hydrocarbons ranging from C.sub.1 to C.sub.4 and thus consists principally of methane admixed with ethane, propane, butane and other saturated, and some unsaturated hydrocarbons. Even though natural gas contains components higher boiling than methane, and such mixtures can be liquified at somewhat higher temperatures than pure methane, the temperatures required for condensation of the admixture is nonetheless too low for natural gas to be liquified and shipped economically. Under these circumstances the natural gas is not even of sufficient value for use as fuel, and it is wasted.
The conversion of methane to a useful liquid fuel would thus be attractive proposition. However, the only industrially feasible technology available today for this conversion is the Mobil process which is operational in New Zealand, where a unique combination of economic factors makes the conversion feasible. Elsewhere however, the energy limitations imposed by this multi-stage process and the additional burden of the Schulz-Flory distribution of products makes the process unsuitable.
Currently there is no known catalyst which provide an economical process capable of converting methane to higher hydrocarbons in a single step catalytic reaction. The Mobil process requires three catalytic reaction steps to transform methane into higher hydrocarbons and is thus of limited use.
The first step of the Mobil process involves the partial combustion of methane to produce carbon monoxide and hydrogen. The second step involves the synthesis of methanol from the carbon monoxide and hydrogen using a copper zinc catalyst at high pressures and low conversions per pass. The third step involves the conversion of the methanol to higher hydrocarbons and water over a zeolite catalyst. Such a multi-stage process is only economically feasible if a rare combination of economic factors are present.
For these reasons process streams which contain methane are usually burned as fuel. The thought of utilizing methane, particularly avoiding the tremendous and absolute waste of a natural resource in the manner outlined above, has challenged many minds; but has produced few solutions.
It would be highly desirable to be able to efficiently convert methane to hydrocarbons of higher molecular weight than methane (hereinafter, C.sub.2 +) particularly admixtures of C.sub.2 + hydrocarbon products which can be economically liquified at remote sites; especially admixtures of C.sub.2 + hydrocarbons rich in ethylene
Ethylene is known to be a particularly valuable chemical raw materials for use in the petroleum, petrochemical, pharmaceutical, plastics and heavy chemicals industries. Ethylene is thus useful for the production of ethyl and ethylene compounds including ethyl alcohol, ethyl ethers, ethylbenzene, styrene, ethylene oxide, ethylene dichloride, ethylene dibromide, acetic acid, polyethylene and the like.
It has been long known that methane, and natural gas could be pyrolytically converted to C.sub.2 + hydrocarbons. For example, methane or natural gas passed through a porcelain tube at moderate red heat will produce ethylene and its more condensed homologues such as propylene, as well as small amounts of acetylene and ethane.
These processes characteristically require considerable heat energy which, most often, is obtained from combustion of the by-product gases. The extreme temperatures make the operation of such processes uneconomical and, of course, serious materials problems are generally encountered. Numerous attempts have been made to catalyze these reactions at lower and more feasible temperatures, but such attempts have met with failure.
In all such processes of converting methane to C.sub.2 + hydrocarbons a partial oxidation mechanism is involved, because hydrogen must be removed either as water, molecular hydrogen or other hydrogen containing specie. Likewise, any other polymerization mechanism in which the methane is converted to C.sub.2 + hydrocarbons products requires a tremendous amount of energy, most often supplied as heat, to provide the driving force for the reactions.
In the past the molecular hydrogen liberated by the reaction has often been burned to provide the necessary process heat. This route has proven an abomination to the production of C.sub.2 + hydrocarbons, but alternate reaction pathways have been little better because they have resulted in the production of large quantities of the higher, less useful hydrogen deficient polymeric materials such as coke, and highly oxidized products such as carbon dioxide and water.
The conversion of methane to higher order hydrocarbons at high temperatures, in excess of about 1200.degree. C. is thus known. These processes are, however, energy intensive and have not been developed to the point where high yields are obtained even with the use of catalysts. And some catalysts that are useful in these processes (e.g., chlorine) are corrosive under such operating conditions.
The catalytic oxidative coupling of methane at atmospheric pressure and temperatures of from about 500.degree. to 1000.degree. C. has been investigated by G. E. Keller and M. M. Bhasin. These researchers reported the synthesis of ethylene via oxidative coupling of methane over a wide variety of metal oxides supported on an alpha alumina structure in Journal of Catalysts 73, 9-19 (1982).
This article discloses the use of single component oxide catalysts that exhibited methane conversion to higher order hydrocarbons at rates no greater than four percent. The process by which Keller and Bhasin oxidized methane was cyclic, varying the feed composition between methane, nitrogen and air (oxygen) to obtain higher selectivities.
West German Pat. No. DE 32370792 discloses the use of a single supported component oxide catalysts. The process taught by this reference utilizes low oxygen partial pressure to give a high selectivity for the formation of ethane and ethylene. The conversion of methane to ethane and ethylene is, however, only on the order of from about four to about seven percent.
Methods for converting methane to higher order hydrocarbons at temperatures in the range of about 500.degree. to about 1000.degree. C. are disclosed in U.S. Pat. Nos. 4,443,644; 4,443,645; 4,443,646; 4,443,647; 4,443,648; and 4,443,648. The processes taught by these references provide relatively high selectivites to higher order hydrocarbons but at relatively low conversion rates, on the order of less than about four percent overall conversion.
In addition to synthesizing hydrocarbons, the processes disclosed in these references also produced a reduced metal oxide which must be frequently regenerated by contact with oxygen. The preferred processes taught by these references entail physically separate zones for a methane contacting step and for an oxygen contacting step, with the reaction promoter recirculating between the two zones.
U.S. Pat. Nos. 4,495,374 and 4,499,322 disclose processes for converting methane to higher order hydrocarbons using an oxidative synthesizing agent containing an alkali metal or compound thereof as a promoter. Both patents indicate that stability of the promoted synthesizing agent is enhanced by the presence of phosphorous.
While the direct dehydrogenative coupling of methane to higher hydrocarbons is thermodynamically infeasible below 1500.degree. K., the oxidative coupling scheme were water is formed in addition to hydrocarbons suffers from no such restraints. However, the best results in the literature to date display modest (.ltoreq.5%) conversions at extremely high temperatures (.gtoreq.710.degree. C.), with a large amount of the methane being oxidized to CO.sub.2 and CO.
Some catalysts containing iron, phosphorus and oxygen are known. Some of these have been used in the oxidation of some hydrocarbons, but not in the transformation of methane to C.sub.2 + hydrocarbons.
There is therefore a strongly felt need for an efficient process for converting methane into useful products, for example, higher hydrocarbons, e.g. ethylene, ethane, or aromatic compounds.